The present invention relates to a method and device for measuring blood flow rate in a blood access. Blood is taken out from the body of a mammal to an extracorporeal blood circuit through a blood access, via needles or a catheter.
There are several types of treatments in which blood is taken out in an extracorporeal blood circuit. Such treatments involve, for example, hemodialysis, hemofiltration, hemodiafiltration, plasmapheresis, blood component separation, blood oxygenation, etc. Normally, blood is removed from a blood vessel at an access site and returned to the same blood vessel or at another location in the body.
In hemodialysis and similar treatments, an access site is commonly surgically created in the nature of a fistula. Blood needles are inserted in the area of the fistula. Blood is taken out from the fistula via an arterial needle and blood is returned to the fistula via a venous needle.
A common method of generating a permanent access site having capability of providing a high blood flow and being operative during several years and even tens of years, is the provision of an arterio-venous fistula. It is produced by operatively connecting the radial artery to the cephalic vein at the level of the forearm. The venous limb of the fistula thickens during the course of several months, permitting repeated insertion of dialysis needles.
An alternative to the arterio-venous fistula is the arterio-venous graft, in which a connection is generated from, for example, the radial artery at the wrist to the basilic vein. The connection is made with a tube graft made from autogenous saphenous vein or from polytetrafluorethylene (PTFE, Teflon). The needles are inserted in the graft.
A third method for blood access is to use a silicon, dual-lumen catheter surgically implanted into one of the large veins.
Further methods find use in specific situations, like a no-needle arterio-venous graft consisting of a T-tube linked to a standard PTFE graft. The T-tube is implanted in the skin. Vascular access is obtained either by unscrewing a plastic plug or by puncturing a septum of said T-tube with a needle. Other methods are also known.
During hemodialysis, it is desirable to obtain a constant blood flow rate of 150-500 ml/min or even higher, and the access site must be prepared for delivering such flow rates. The blood flow in an AV fistula is often 800 ml/min or larger, permitting delivery of a blood flow rate in the desired range.
In the absence of a sufficient forward blood flow, the extracorporeal circuit blood pump will take up some of the already treated blood entering the fistula via the venous needle, so called access or fistula recirculation, leading to poor treatment results.
The most common cause of poor flow with AV fistulas is partial obstruction of the venous limb due to fibrosis secondary to multiple venipunctures. Moreover, stenosis causes a reduction of access flow.
When there is a problem with access flow, it has been found that access flow rate often exhibit a long plateau time period with reduced but sufficient access flow, followed by a short period of a few weeks with markedly reduced access flow leading to recirculation and ultimately access failure. By constantly monitoring the evolution of the access flow during consecutive treatment sessions, it is possible to detect imminent access flow problems.
Several methods have been suggested for monitoring recirculation and access flow. Many of these methods involve injection of a marker substance in blood, and the resultant recirculation is detected. The methods normally involve measurement of a property in the extracorporeal blood circuit. Examples of such methods can be found in U.S. Pat. Nos. 5,685,989, 5,595,182, 5,453,576, 5,510,716, 5,510,717, 5,312,550, etc.
Such methods have the disadvantage that they cannot detect when the access flow has decreased to such an extent that recirculation is at risk, but only when recirculation prevails. Moreover, it is a drawback that injection of a substance is necessary.
A noninvasive technique that allows imaging of flow through AV grafts is color Doppler ultrasound. However, this technique requires expensive equipment.
The measurement of access flow rate necessitates the reversal of the flows in the extracorporeal circuit. A valve for such reversal is shown in i.a. U.S. Pat. Nos. 5,605,630 and 5,894,011. However, these valve constructions comprise dead ends in which blood may stand still for a long time and coagulate, which is a drawback.
An object of the present invention is to provide a method and a device for measuring the access flow rate without interfering with the blood and without injecting a substance in blood.
Another object of the invention is to provide a method and a device for measuring access flow rate without measuring on the blood in the extracorporeal blood circuit or in the access or blood vessel.
According to the invention, it is required to reverse the blood flow through the access. Thus, a further object of the invention is to provide a valve for reversing the blood flow.
A still further object of the invention is to provide a method for determining when the blood flow rate is so small that risk for recirculation prevails.
These objects are achieved with a method and an apparatus for estimating fluid flow rate (Qa) in a fluid flow access, comprising removing a first fluid flow from said access at a removal position to an external flow circuit comprising a dialyzer having a semipermeable membrane, said first fluid flow passing along said membrane at one side thereof and a dialysis fluid being emitted from the other side thereof, and returning said first fluid flow from said external flow circuit to said access at a return position downstream of said removal position, measuring a first variable which is essentially proportional to a concentration (Cd norm) of a substance in said dialysis fluid emitted from the dialyzer, reversing the removal position with the return position and measuring a second variable which is essentially proportional to the concentration (Cd rev) of said substance in said dialysis fluid in the reversed position; and calculating the fluid flow rate (Qa) in said flow access from said measured concentrations.
Preferably, the calculation of the fluid flow rate in said flow access takes place by calculating the ratio between the first and the second variable and using the formula: Cd norm/Cd rev=1+K/Qa, in which Cd norm and Cd rev are values proportional to the concentrations of said substance in the dialysis fluid in the normal and reversed positions, respectively, and K is the clearance of the dialyzer and Qa is the access flow rate.
The blood flow access may be in a mammal for obtaining access to a blood vessel, such as a hemodialysis access in the nature of an arterio-venous shunt or fistula. In the latter case, the dialyzer clearance K is replaced by the effective dialyzer clearance Keff obtained by taking into account a cardiopulmonary recirculation and in the normal position.
The substance is preferably selected from the group of: urea, creatinine, vitamin B12, beta-two-microglobuline and glucose, or may be an ion selected from the group of: Na+, Clxe2x88x92, K+, Mg++, Ca++, HCO3xe2x88x92, acetate ion, or any combination thereof as measured by conductivity; and wherein said concentration is measured as the concentration difference between the outlet and the inlet of the dialyzer, if applicable.
It is possible to measure the actual concentration of the substance. However, since only the ratio between the concentrations in the normal and the reversed position, respectively, is needed, it is possible to measure a value which is proportional to the concentration of said substance, whereby said value is used in place of said concentration. Said property may be the blood concentration of said substance in the external circuit, either before or after the dialyzer. Alternatively, the relative whole body efficiency (Kwh/V) may be used, as explained in more detail below.
The effective clearance Keff may be obtained by the equation Keff=Qd*Cd/Cs, where Qd is the flow of dialysis fluid emitted from the dialyzer, Cd is the concentration of said substance in said dialysis fluid and Cs is the concentration of said substance in systemic venous blood.
A method of measuring the concentration (Cs) of said substance in systemic venous blood comprises the steps of: stopping the blood flow in the external flow circuit for a time period sufficient to allow the cardiopulmonary circulation to equalize; starting the blood flow in the external flow circuit with a slow speed to fill the arterial line with fresh blood before the measurement; and measuring the equalized concentration of said substance in the dialysis fluid at a low dialysate flow rate or at isolated ultrafiltration. It is advantageous to make the measurement of the effective clearance at the initiation of the treatment.
The concentration (Cs) of said substance in systemic venous blood may be estimated by: calculating a whole body mass of urea (Murea) in the body of the patient, estimating or measuring the distribution volume (V) of urea in the body of the patient; and estimating the concentration (Cs) of said substance in the blood by dividing the whole body mass of urea with the distribution volume. In this way, the mean concentration of urea in the whole body is obtained. However, the mean concentration in the whole body is slightly higher than the urea concentration in the systemic blood, except at the start of the treatment. Thus, this calculation should preferably be done or be extrapolated to the start of the treatment.
It is possible to discriminate between the condition when access or fistula recirculation has developed and not. A method for that purpose would be: changing the blood flow rate (Qb); monitoring the concentration of said substance in the dialysate emitted from the dialyzer; and detecting a possible fistula recirculation in the normal position by correlating a change in said concentration to said change of the blood flow rate.
Preferably, the blood flow rate is decreased and a corresponding decrease in the urea concentration is monitored, and the absence of such a decrease being indicative of fistula recirculation.